In order to improve an SN ratio of a read signal read from the recording medium on which the data is recorded at high density, there is known a technology by which a filtering process for emphasizing high frequencies is performed on the read signal, for waveform equalization. In particular, according to a patent document 1, it discloses the technology by which the high frequencies can be emphasized without any intersymbol interference by performing the filtering process after amplitude limit is performed on the read signal (a technology about a so-called limit equalizer).    Patent document 1: Japanese Patent No. 3459563